Electron Density and Temperature Measurement by Stark Broadening in a Cold Argon Arc- Plasma Jet at Atmospheric Pressure This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2009 Plasma Sci. Technol. 11 560 (http://iopscience.iop.org/1009-0630/11/5/09) Download details: IP Address: 128.59.62.83 The article was downloaded on 17/04/2013 at 12:03 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Electron Density and Temperature Measurement by Stark Broadening in a Cold Argon Arc-
Plasma Jet at Atmospheric Pressure
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2009 Plasma Sci. Technol. 11 560
(http://iopscience.iop.org/1009-0630/11/5/09)
Download details:
IP Address: 128.59.62.83
The article was downloaded on 17/04/2013 at 12:03
Please note that terms and conditions apply.
View the table of contents for this issue, or go to the journal homepage for more
Home Search Collections Journals About Contact us My IOPscience
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
Abstract Determination of both the electron density and temperature simultaneously in a cold
argon arc-plasma jet by analyzing the Stark broadening of two different emission lines is presented.
This method is based on the fact that the Stark broadening of different lines has a different
dependence on the electron density and temperature. Therefore, a comparison of two or more line
broadenings allows us to diagnose the electron density and temperature simultaneously. In this
study we used the first two Balmer series hydrogen lines Hα and Hβ for their large broadening
width. For this purpose, a small amount of hydrogen was introduced into the discharge gas. The
results of the Gigosos-Cardenoso computational model, considering more relevant processes for the
hydrogen Balmer lines, is used to process the experimental data. With this method, we obtained
reliable electron density and temperature, 1.88 × 1015 cm−3 and 13000 K, respectively. Possible
sources of error were also analyzed.
Keywords: plasma jet, Stark broadening, Gigosos-Cardenoso model
PACS: 52.70.-m, 52.25.-b
1 Introduction
When the electron temperature is known, the Starkbroadening of certain lines spontaneously emitted byplasmas can be used as a convenient way to determinethe electron density. This method has been used fordecades. Recently this method has drawn more andmore attention in the diagnosis of atmospheric pres-sure plasmas, because other traditional methods have agreat deal of difficulty under such conditions. For exam-ple, the Langmuir probe, which is widely used for lowpressure plasmas, is less useful for plasmas generatedat atmospheric pressure. Another well-known method,Thomson scattering, is rarely adopted in plasma diag-nosis because it involves a complicated experimentalsystem and high cost [1,2].
In this study, we used a so-called crossing-pointmethod to measure the electron density ne and temper-ature Te simultaneously by analyzing the Stark broad-ening of different Balmer series lines of hydrogen. Ac-cording to the theory of spectrum, Stark broadening ofdifferent spectral lines depends on the electron densityand temperature in different ways. If we know the re-lationship of not less than two spectral lines from thesame plasma at the same time, it is possible to mea-sure the electron density and temperature simultane-ously. This method was proposed first by TORRES Jet al. and has been used successfully [3,4]. In fact, arapid and approximate estimation of the magnitudes ofthese two parameters can be sufficient for a large num-ber of cases, where only a rough value is needed. Sothe advantages and disadvantages are reasonably bal-anced in this discharge diagnostic method compared toother accurate but more complicated ones. In addition,
in order to find the relationship between Stark broad-ening and other parameters of plasma, different theo-ries have been developed. Here, in data processing, weadopted the results of the Gigosos-Cardenoso compu-tational model [5,6] which is the most important andaccurate one for the hydrogen Balmer series emissionlines. This model considers relevant processes happen-ing in the plasma, more than any other theory did.
Recently non-equilibrium plasma jet at atmosphericpressure has attracted much attention for its greatprospects in many fields of application. It can pro-vide an alternative which is easier to set up, more eco-nomical and convenient to operate compared to con-ventional low pressure plasmas [7]. In our laboratory, asimple discharge device referred to as a cold arc-plasmajet was used to generate an atmospheric pressure non-equilibrium plasma (a typical two temperature plasma),which can be used for surface modification, sterilizationand so on. This kind of plasma has many advantagesdue to its high electron density and temperature andlow gas temperature. These characteristics of the coldplasma jet are crucial in treating thermally sensitivematerials. This paper is organized as follows. Section2 briefly describes the experimental arrangement. Theexperimental results and some discussion are presentedin section 3. Section 4 presents the conclusions.
2 Experimental setup
Fig. 1 is a schematic diagram of the experimentalsetup. Outside of the discharge device is a stainlesssteel cylinder of 14 mm in external diameter and 3 mmin thickness, which was used as the cathode connected
ZHOU Qiuping et al.: Electron Density and Temperature Measurement by Stark Broadening
to the ground. At the end of the cylinder is a hole witha diameter of 5 mm, from which discharging gases isintroduced into the chamber. The anode is a copperrod of 2 mm in diameter, which was inserted into thecenter of the stainless steel cylinder. In order to re-strict the arc discharge to a small area, the copper rodis partially covered (shown in Fig. 1) by a quartz tube(1 mm in thickness) with an inner diameter as the sameas the diameter of the copper rod. Between the cathodeand anode, a ceramic disk was used as the insulatingbarrier. In our experiment, the AC power supplier wasa commercially available transformer with continuouslytunable output voltage and frequency. Once argon wasintroduced through the gas inlet and a high AC volt-age was applied, a discharge was ignited in the gapbetween the electrodes and a plasma jet was generatedsynchronously (the nozzle is 3 mm in diameter) at thetip of the stainless steel cylinder, as shown in Fig. 1.
Fig.1 Schematic diagram of experimental setup
In the experiment, the emission spectrum fromthe plasma jet was analyzed by an AvaSpec-2048monochromator (with a fiber length of 2 m) equippedwith a holographic grating of 1800 lines/mm and a slitof a width of 10 µm. The light from the discharge wasfocused onto the entrance of the spectrometer using aquartz lens. The emission spectrum of the discharge ina range of 200∼ 715 nm was recorded after subtract-ing background images as the discharge was off. Theplasma jet generated by our discharge device had such aslender profile that it was difficult to analyze the emis-sion from different radial positions. Thus, our measure-ments were effective for a given z axial position whichwas measured from the tip of the discharge device noz-zle.
The argon plasma in our experiment was gener-ated by supplying a peak-to-peak voltage of 3.6 kV ata frequency of 17.6 kHz and a flow rate of argon of1200 L· h−1. Fig. 2 shows the discharge voltage andcurrent waveform recorded by oscilloscopes. The totaloutput power was calculated to be about 14 Watts bythe following formula
P =1T
∫ T
0
u(t)i(t)dt. (1)
Moreover, to observe the hydrogen Balmer emissionlines from the plasma jet, a small amount of hydrogen(about 1%) was introduced into the discharge chamber.This quantity should be enough for measuring the in-tensities of the Hα and Hβ lines without disturbing thedischarge significantly. Fig. 3 shows the spectrogramcorresponding to the position z = 1 mm.
Fig.2 Discharge voltage and current waveform recorded
by oscilloscopes
Fig.3 Emission spectrum in the range of 200∼ 715 nm
(z =1 mm) by supplying a peak-to-peak voltage of 3.6 kV
and a frequency of 17.6 kHz
3 Results and discussion
A specific spectral line emitted spontaneously byplasmas can be broadened due to different mechanismsand its total shape is due to the combined contributionof all the causes. Each mechanism can make a shift inthe energy levels of the emitting atoms and the rela-tive importance of these broadenings depends on theplasma conditions [3]. In our case for the cold plasmawith a moderate electron density and temperature, therelevant sources of broadening necessary to consider in-clude the Stark effect, Doppler broadening and Van der
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Plasma Science and Technology, Vol.11, No.5, Oct. 2009
Waals broadening [8]. Doppler broadening is caused bythe random thermal motion of the emitting atoms andits full width at half maximum (FWHM) ∆λD is relatedto the gas temperature Te
[9],
∆λD(nm) = 7.16× 10−7λ0
√Tg
M, (2)
where M is the mass of the emitters expressed in atomicmass units and λ0 is the central wavelength of the line innm. Van der Waals broadening should not be neglectedfor a low ionization degree and a high density of neutralatoms. This broadening results from the dipole inter-action between excited atoms and the induced dipole ofthe neutral perturbers, and has a total FWHM givenby [3]
∆λWaals(nm) = Ki(Tg/µ)3/10N, (3)
where µ is the reduced mass of the emitter-perturberpair, N is the neutral density which can be obtainedfrom the gas temperature Tg via the ideal gas law, andKi is a constant depending on the spectral line and theemitter polarizability. Finally, the instrumental broad-ening introduced by the optical system used in the ex-periment is another important external contribution tothe total spectral line profile.
The total spectral line shape is a convolution of allthese broadenings. Both the Stark and Van der Waalsbroadening can be well fitted by a Lorentz function, andtheir convolution can also be expressed by a Lorentzfunction with a total FWHM [10]
∆λL = ∆λS + ∆λWaals, (4)
where ∆λS is the FWHM of the Stark broadening. TheDoppler broadening and the instrumental broadeningcan be supposed to follow a Gaussian function and theirconvolution can also be presented in a Gaussian func-tion. Its total FWHM can then be expressed as
∆λ2G = ∆λ2
D + ∆λ2instrument, (5)
with ∆λinstrument the FWHM of the instrumentalbroadening. Finally, their combined contribution leadsto a Voigt function which is used to fit the total profileof the spectral lines. So, the first step in analyzing theexperimental data is to make a deconvolution to obtainthe Lorentz part from the total Voigt profile and thisoperation can be carried out by some software easily.Then, according to Eq. (4), we can conveniently obtainthe FWHM of the Stark broadening by subtracting theVan der Waals broadening part from the total Lorentzbroadening as ∆λWaals is known. In our case for the hy-drogen Balmer lines Hα and Hβ , Eq. (3) has a simpleform as follows [11,12]
∆λαWaals =
3.5T 0.7
g
, (6)
∆λβWaals =
1.8T 0.7
g
. (7)
In the spectrogram of the argon plasma jet withintroduced hydrogen, the Hα and Hβ lines could be
observed distinctly, as shown in Fig. 3. Their Voigt-function fitting results are shown in Fig. 4(a) and(b). In order to calculate ∆λα
Waals and ∆λβWaals from
Eqs. (6) and (7), gas temperature Tg is needed. For acold arc-plasma jet under similar conditions [13], thisTg value is taken as room temperature, namely thegas temperature Tg was measured, by means of rota-tional band spectroscopy in an experiment of photonsscattered by the cold plasma jet, as about 290 K. Inour case, we assumed that the gas temperature was300 K as an appropriate value, and then the Van derWaals broadening of the Hα and Hβ lines can be cal-culated, according to Eqs. (6) and (7), to be 0.0646 nmand 0.0332 nm, respectively. Finally, the FWHM val-ues for the Stark broadening of the two spectral linesare 0.0878 nm and 0.3399 nm.
Fig.4 (a) Normalized spectral line Hα and corresponding
Voigt-function fitting results, (b) Normalized spectral line
Hβ and corresponding Voigt-function fitting results
When both the electron density ne and temperatureTe are being determined by Stark broadening, the re-sults of the Gigosos-Cardenoso computational modelwere used. GIGOSOS and co-workers have publishedsome useful tables of data [5] which show the depen-dence of ∆λS on electron densities ne and electron tem-peratures Te for the hydrogen Balmer lines. Based onthese tabulated data and a given specific FWHM valuefor Stark broadening, a curve describing the relation-ship between the electron density and electron temper-
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ZHOU Qiuping et al.: Electron Density and Temperature Measurement by Stark Broadening
ature can be obtained by an interpolation process. If weonly draw the two curves of the Hα and Hβ lines in thesame coordinate system, ne and Te can be simultane-ously determined from the coordinate of their crossingpoint. As shown in Fig. 5, ne and Te were found to be1.88 ×1015 cm−3 and 13000 K, respectively.
Uncertainty in the crossing-point method may becaused by several factors, including experimental er-ror in the measurement of the spectral lines, the fittingprocess and possible computational error due to theGigosos-Cardenoso model. Among these factors the er-ror from the Hβ line measurement and fitting process iseven more serious than that from the Hα line measure-ment. This is because its intensity is very low relativeto the background signal, and furthermore, this linealso contains some noise which should be removed by asmoothing process before the data fitting. Another factwhich should be stated is that the error in the electrontemperature is more serious than that in the electrondensity due to the relative shape of the curves. In fact,the FWHM of the Stark broadening is only a functionof the electron density to some extent for a certain case,so the curve corresponding to the Hβ line is relativelyflat as shown in Fig. 5.
Fig.5 Relationship between electron density and
electron temperature for FWHM(Hα)= 0.0878 nm and
FWHM(Hβ)= 0.3399 nm
4 Conclusions
Reliable electron density and temperature values ina cold argon arc-plasma jet at atmospheric pressurewere obtained simultaneously by using the crossing-point method. Compared to other methods suitable
for similar conditions, this method is easier and moreconvenient. The only prerequisite is that not less thantwo different spectral line shapes should be known atthe same time to create a crossing point. On the otherhand, in order to find the relationship between Starkbroadening, electron density and temperature, the re-sults of the Gigosos-Cardenoso computational modelwere used. This model takes the ion dynamics intoaccount and is the most accurate model for hydrogenBalmer lines so far. For this purpose, a small amountof hydrogen is needed for non-hydrogen plasmas. The-oretically speaking, we can adopt any two spectral linesfrom a pure argon plasma to generate a crossing point.However there has not yet been a widely accepted, ac-curate theoretical model for argon spectral lines. In ad-dition, other plasma parameters of the same dischargedevice (rotational temperature and vibrational temper-ature) will be determined in our future research.
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